The GP1,2 spike proteins of filoviruses (marburgviruses and ebolaviruses)
mediate viral cell-surface attachment, membrane fusion, and entry into cells
expressing the unknown filovirus receptor(s). Here, it is shown that a 151
amino-acid fragment of the Lake Victoria marburgvirus GP1 subunit (residues
38-188), fused to the Fc region of human IgG1, bound filovirus-permissive cell
lines more efficiently than full-length GP1. An analogous 148 amino-acid
fragment of the Zaire ebolavirus GP1 subunit (residues 54-201) similarly bound
the same cell lines more efficiently than a series of longer GP1-truncation
variants. Neither the marburgvirus GP1 fragment, nor that of ebolavirus, bound
to filovirus-resistant lymphocyte cell lines thought not to express the
filovirus receptor. Both Lake Victoria marburgvirus 38-188-Fc and Zaire
ebolavirus 54-201-Fc specifically inhibited the replication of infectious
Zaire ebolavirus, as well as transduction of filovirus-permissive cells by
gammaretroviruses pseudotyped with either the Lake Victoria marburgvirus or
the Zaire ebolavirus GP1,2 spike protein. Similarly, GP1-Fc fusion fragments
of Côte d’Ivoire ebolavirus, Reston ebolavirus, and Sudan ebolavirus,
corresponding to Zaire ebolavirus GP1 residues 54-201, inhibited
gammaretroviruses pseudotyped with Lake Victoria marburgvirus GP1,2. These
studies identified the receptor-binding regions (RBRs) of marburgviruses and
ebolaviruses, and demonstrated that all filoviruses utilize at least one
common receptor. In addition to the GP1,2 spike glycoprotein, ebolaviruses,
but not marburgviruses, express two secreted glycoproteins, sGP and ssGP, from
the GP gene by cotranscriptional editing. All three proteins have identical
N-termini that include residues 54-201. However, it is shown that neither sGP-
Fc nor ssGP-Fc binds to filovirus-permissive cells. Both proteins were unable
to inhibit transduction of such cells by gammaretroviruses pseudotyped with
the Lake Victoria marburgvirus GP1,2 spike protein, indicating that they do
not bind to the filovirus receptor. Instead, it is shown that Fc-conjugated
Δ-peptide, which is a short C-terminal cleavage product of sGP bearing no
sequence similarity to the filoviral RBRs, inhibited pseudotyped
gammaretroviruses and infectious Zaire ebolavirus specifically and in a dose-
dependent manner. Δ-Fc derived from Côte d’Ivoire, Sudan, and Zaire ebolavirus
sGP inhibited Lake Victoria marburgvirus GP1,2 spike protein-mediated entry
and replication of infectious Zaire ebolavirus comparably or better than Zaire
ebolavirus 54-201-Fc. Interestingly, Δ-Fc derived from sGP of Reston
ebolavirus, thought to be the only filovirus apathogenic for humans, had
little or no effect. These data suggest that Δ-peptides modulate filovirus
cell entry and may be important virulence factors. Last, the immunogenic
properties of filoviral RBR-Fcs were evaluated in a lethal ebolavirus mouse
model. C57/BL6 mice were immunized on days 0, 21, and 35 with Zaire ebolavirus
54-201-Fc + RIBI adjuvant or Lake Victoria marburgvirus 38-188-Fc + RIBI
adjuvant and challenged with mouse-adapted Zaire ebolavirus on day 62. All
mice immunized with Zaire ebolavirus 54-201-Fc survived otherwise lethal
challenge without showing any signs of disease. Half of the mice immunized
with Lake Victoria marburgvirus 38-188-Fc also survived otherwise lethal Zaire
ebolavirus infection. Sera collected from mice immunized with Zaire ebolavirus
54-201-Fc or Lake Victoria marburgvirus 38-188-Fc neutralized Zaire ebolavirus
infection of Vero E6 cells, and strong cytotoxic T-lymphocyte responses were
detected in mice immunized with either RBR-Fc. This is the first report of a
cross-protective filovirus candidate vaccine.